Excerpts from Books and Wikipedia
"The Louisiana Purchase of 1803, which opened up huge areas west of the Mississippi to U.S. settlement, raised the divisive question of whether slavery should be permitted in territories that joined the union as new states. . . . Missouri's admission as a slave state was balanced by making the northern part of Massachusetts the new non-slave state of Maine. . . . President James Monroe had doubts about the compromise, but approved it to stop the Union division. In 1857, the Supreme Court declared the Missouri compromise unconstitutional, which helped make the Civil War inevitable." [Furtado: 1001 Days]"Although the Panic of 1819 did not prevent the reelection of Monroe, another crisis occurred simultaneously that gave the administration a bad scare: the Missouri Controversy. By 1819, enough settlers had crossed the Mississippi River that Missouri Territory could meet the usual population criterion for admission to the Union. . . . Before it was over, not just the extension of slavery on the frontier but the existence of slavery throughout the whole Union would be challenged. . . . The Missouri Controversy prefigured the coming forty-five years of sectional conflict." [Howe: What Hath God Wrought, p. 147-8]